Environmental Benefits of Tile Drainage - Literature … · Environmental Benefits of Tile Drainage Page: 1 Environmental Benefits of Tile Drainage - Literature Review - by: Heather

Environmental Benefits of Tile Drainage- Literature Review -

by: Heather Fraser and Ron Fleming, P. Eng.,Ridgetown College - University of Guelph

prepared for: LICO - Land Improvement Contractors of Ontario

October, 2001

Table of Contents

1.0 Introduction.........................................................................................................................1

2.0 Agronomic Benefits............................................................................................................2

3.0 Economic Benefits of Tile Drainage ..................................................................................2

4.0 Environmental Benefits of Tile Drainage ..........................................................................34.1 Hydrology..............................................................................................................3

a) Storage Space..............................................................................................4b) Infiltration vs surface runoff .....................................................................4c) Peak flows reduced.....................................................................................5d) Stream flow.................................................................................................6e) Groundwater ...............................................................................................7

4.2 Water Quality ........................................................................................................7a) Sediment ....................................................................................................7b) Pesticides ....................................................................................................8c) Manure application.....................................................................................8d) Nutrients .....................................................................................................9

5.0 Management Systems ......................................................................................................115.1 Nutrient Management Planning...........................................................................115.2 Water table management .....................................................................................115.3 Constructed wetlands...........................................................................................125.4 Other ....................................................................................................................13

6.0 Summary and Conclusions..............................................................................................13

7.0 References.........................................................................................................................15

Environmental Benefits of Tile Drainage Page: 1

Environmental Benefits of Tile Drainage- Literature Review -

by: Heather Fraser and Ron Fleming, P. Eng.

1.0 Introduction

Agricultural land drainage has been practiced for millennia. Greek and Egyptiancivilizations relied on surface drainage to preserve cropland from being damaged by floodwaters. Since then, agricultural drainage has continued to change and develop throughoutthe years to now include subsurface drainage (Donnan 1976). England boasts to have laidthe first cylindrical tile drains in 1810. 1844 marks the time when the first clay tile wasreported to have been laid in Canada, near Bowmanville, Ontario (Irwin, 19__). In theUnited States, widespread use of concrete tiles occurred about 1900 (Donnan 1976).Subsurface drainage improved land for agricultural production, and was a way to controldiseases carried by the mosquitoes and black flies living in wet areas. The USDA (1955)claims that land drainage facilitated settlement in North America.

A more recent look at land drainage reveals that, out the 170 million hectares (ha) ofcropland in the U.S., 45 million ha have benefited from some form of improved drainage.15.3 million ha (34%) of that has been tile drained (Skaggs et al. 1994). In Ontario, thereare 3.5 million ha of land classified as cropland (OMAFRA 1996). It is estimated that 1.5million ha (43%) has been tile drained, and that an additional 1.5 million could benefitfrom tile drainage within the next 30 years (Vanderveen 2001). Though tile drainage hasbeen practiced for over 150 years in some of these areas, there is still much land thatcould benefit from tile drainage.

The term tile drainage refers to a subsurface conduit for removing excess water from thesoil. This pipe can be made of fired clay, concrete, or more popularly, perforatedcorrugated plastic. The term agricultural drainage refers to both surface and subsurfacedrainage. The purpose of this report is to provide an overview of research conducted ontile drainage. It will also provide a thorough list of references to consult for furtherreading. The outline of the report is as follows:

< agronomic benefits< economic benefits< hydrology< water quality< innovative management< conclusions/recommendations


2.0 Agronomic Benefits

The agronomic reasons for having tile drains installed into farm land are many. Tiledrainage can affect the physical properties of the soil. The removal of excess water resultsin better aeration and corresponding microbial activity, improved soil porosity and tilthand overall better soil structure (Hill 1976; Gardner et al. 1994). Because dry soil warmsup faster than wet soil, tile drainage also promotes warmer spring soil temperatures whichcan lead to earlier spring sowing and germination of seeds (Plamenac 1988; Liefers andRothwell 1987).

By removing excess water from the upper layers of the soil more quickly than undrainedsoils, tile drainage can improve the trafficability of soil (Geohring and Steenhuis 1987;Madramootoo et al. 1997; and Bailey 1979). A study conducted in British Columbia on anaturally moderately poor to poorly drained silty-clay loam found that subsurfacedrainage could remarkably advance soil trafficability by more than 60 days (Chieng et al.1986). A more modest advancement was seen by Aldahagh and Beer (1975), who foundspring workability could be increased by 16 days on a poorly draining Webster silty-clayloam. A modeling study by Wendte et al. (1978 ) predicted similar results. Lengthenedgrowing seasons are important, especially in climates where growing seasons are limited(Madramootoo et al. 1997). Heavy machinery use and tillage on wet soil can result in soilcompaction, which damages the soil structure (Wind 1976). Tile drainage reduces the riskof this damage.

3.0 Economic Benefits of Tile Drainage

Tile drainage is economically beneficial to the farmer. It can allow higher value crops,such as fruits and vegetables, to be planted where they could not otherwise be profitablygrown. Tiling can increase the yield for a variety of different crops, including: wheat, corn,soybeans, sugar beets, sunflowers, sugar cane, citrus and forages (Bolton et al. 1982;Warin et al. 1998; Geohring and Snyder 1983; Colwell 1978; Buscaglia et al. 1994;Geohring and Steenhuis 1987; Carter 1987; and Plamenac 1988). For example, Colwell(1978) reported yield increases of 35, 32, 48 , 47 and 27% for grain corn, soybeans, wheat,oats and hay (see Table 1). Finally, because machinery works more efficiently on driersoil, tile drainage reduces labour hours (Plamenac 1988) and helps to minimize fossil fuelconsumption and associated costs (Madramootoo et al. 1997; Wind 1976).


Table 1 B Effect of tile drainage on yields of five different crops

Average Yieldbefore tiledrainage

Average yieldafter tiledrainage

Yield increaseCrop

tonne/ha tonne/ha tonne/ha %

Grain corn 4.14 5.58 1.44 34.8

Soybeans 1.96 2.59 0.63 32.1

Wheat 1.77 2.61 0.84 47.5

Oats 1.60 2.35 0.75 46.9

Hay 4.10 5.20 1.10 26.8

(from Colwell 1978)

In Ontario, after analyzing 51 years of data on subsurface drainage, an after-tax analysisindicated that government benefited through increased tax revenue. Higher incomefarmers would invest profits, which added to the tax revenue as investment income (vanVuuren and Jojani 1986). Not all areas of land, however, will realize economic benefitsfrom tile drainage installation (Found et al. 1976). An appropriate economic analysisshould be completed prior to installation.

4.0 Environmental Benefits of Tile Drainage

At the turn of the 21st century, it is not enough for the farmer to ensure that agriculture isfulfilling its productivity and economic demands. It is important that the agriculturalsector manages itself in an environmentally responsible manner. This section will addressthe various ways in which land with tile drainage can affect the environment.

4.1 Hydrology

Comparisons in this report will be made between tiled and untiled agricultural land.Natural, undrained land, behaves differently - for example, peak runoff rates as well assediment and pollutant loading are lower in natural systems than agricultural systems(Skaggs et al. 1994). Bearing in mind that agricultural production is essential to societyand the economy, this section will review water behavior in agricultural areas only.


a) Storage SpaceThe literature agrees that tile drained fields can offer more temporary storage space forwater than their undrained equivalents (Van Vlack and Norton 1944; Mason and Rost1951; Skaggs and Broadhead 1982; and Irwin and Whitely 1983; ). Skaggs andBroadhead (1982) observed five different storm events. They noted that in these events,subsurface drainage increased storage capacity in the soil by continually removing excess,or "loose" water, from the soil profile. This Aloose@ water (also called gravitational water)is not available for use by plant roots. It fills the soil pores normally occupied by air andleads to drowning of crops. Plant roots use Acapillary@ water, which is held to soil particlesby surface tension. Soils with tile drainage were found to have a greater storage capacitythan naturally well-draining soils that did not have tile drainage. Irwin and Whitely (1983)reviewed literature from the US, UK, Europe and Canada. They found that under"drained" conditions, it should take the water table three to four days to fall to drainagedepth. In contrast, undrained fields may take several weeks for evaporation alone to lowerthe water table to a similar depth. If there is an intervening rain, it will take longer.

Further, some soils drained with tiles may actually have a higher capacity to store waterbecause tile drainage improves soil structure. Better soil structure means that the soil ismore porous, and is therefore better able to store water (Gardner et al. 1994).

b) Infiltration vs surface runoffBecause tile drained soils can have a higher capacity to store added moisture, more wateris able to infiltrate into the soil profile, thus reducing surface runoff volumes. Extensivereview articles covering North American, European and British literature (Baker andJohnson 1976; Hill 1976; Irwin and Whitely 1983; Belcher and Fogiel 1991; and Thomaset al. 1995), as well as other individual studies (Van Vlack and Norton 1944; Mason andRost, 1951; Watelet and Johnson 1999 ) have confirmed that tile drainage reduces surfacerunoff. For example, in western Oregon, Istok and Kling (1983) observed that when tiledrainage was installed in a silt loam watershed with slopes ranging from 0-15%, watershedrunoff yield was reduced by 65%.

Reduced surface runoff can result in decreased soil, chemical and nutrient losses from afield, though the losses are dependant on the timing of the runoff event with respect to thetime of application and protective cover crop. It can also decrease peak flows and totalvolumes lost from the watershed, as well as increase the time between the beginning of arain event and the peak flow ("lag time"). These topics will be discussed in later sections.


c) Peak flows reducedGood subsurface drainage significantly reduces peak flow volumes. Peak flow refers tothe greatest amount of flow resulting from the total of surface runoff and subsurfacedrainage. Because increased storage capacity allows more water to infiltrate, the soil actsas a buffer for rainfall and spreads the runoff over a longer period of time (Mason andRost, 1951; Larson et al. 1980).

Skaggs and Broadhead (1982) found that conditions prior to precipitation affected howwell tile drained soil buffered peak flows. By monitoring five different storm events, theyfound that good subsurface drainage significantly reduced peak flows. Subsurfacedrainage increases storage capacity in the soil by continually removing water from the soilprofile. If the soil was very wet prior to precipitation, peak flow was reduced by 20%, butwhen prior conditions were dry, a reduction of up to 87% was observed. It was alsoemphasized that there is an interaction between subsurface and surface drainage. Whensubsurface drainage was good, improvements to the surface drainage system made littledifference to peak flow rates. However, when subsurface drainage was poor, improvingthe surface drainage increased the peak flows.

Natho-Jina et al. (1986) found that peak flows were significantly influenced by the soilmoisture condition. When soil was dry before a rain event, there was a relatively smallamount of tile flow. The water table had been lowered by the subsurface drainagesystem, so there was more space in the soil profile available for storage. They also foundthat the subsurface flow hydrograph was flatter and longer compared to the surface runoffhydrograph. However, most of the water removed from the field was from the tiledrainage. The Quebec study was completed on a silt-loam soil having a slope of 2%.

Larson et al. (1980) also found that soil moisture conditions affected peak flow. Whensoil moisture levels were low, subsurface drainage produced only small increases (if any)in storm runoff volumes, compared to a surface inlet. However, when soil moistureconditions were high, significant increases in storm runoff volume occurred fromsubsurface drainage but the discharges were maintained over a longer period of time. Theresearchers found that the presence of subsurface drainage produced a significant increasein the annual total flow volume (total of surface runoff and subsurface flow), compared tosurface drainage only.

Konyha et al. (1992) also found that total outflow from the field increased, by 40 mm(10%) when compared to surface drainage only. However, the peak runoff rate wasreduced from 101 to 28 mm/day. The volume of surface runoff from the field decreasedby 192 mm (66%). This modeling study was conducted on well- to poorly-drainedWadasa muck soil in North Carolina. When McLean and Schwab (1982) modeled total


runoff at a watershed scale, peak flow runoff rate reductions of up to 18% during thegrowing season and 11% in the non-growing season were seen. The study gathered longterm data from 0.2 ha plots under four different drainage regimes. On average, peak flowrates from the field were reduced by an average of 32%, ranging from 7 to 77%, comparedto undrained plots. For storms that produced high rates of surface runoff, peak flow rateswere reduced by 50% with tile drainage.

Istok and Kling (1983) observed that when tile drainage was installed in a silt loamwatershed with slopes ranging from 0-15%, watershed runoff was reduced by 65% andsediment yield was reduced by 55%. The hydrology of this western Oregon watershedalso changed so that the lag time increased - drainage water moved through the watershedmore gradually, which is generally desirable.

Many of the studies mentioned in this review have been conducted on relatively flatsurfaces. Parkinson and Reid (1986) determined that slope plays a large role in theefficiency of tile drainage. The site was a heavy clay soil with slopes ranging from 3.6 to5%. Drainage efficiency, peak drain discharge and flood lag time all decreased as slopeincreased - i.e. as more surface runoff occurred.

Soil type plays a major role in tile drainage volume and rate. After comparing thedrainage from four sites with four different soil types, Clark et al. (1988) found thatdrainage response of heavy clays is slow initially but increases rapidly once a route-wayhas been established. Cracks which mainly form during dry periods make theroute-ways. Other soil types have a more constant flow response. The land slope in thisexperiment was minimal, ranging from 1.5 to 2.0%.

Tile drainage spacing affects the quantity of tile discharge. Hoover and Schwab (1976)found that tile spacings of 9.1 m (30 feet) increased the tile flow discharge by 50%compared to spacings of 15.2 m (50 feet). Schwab et al. (1961) compared 9.1 m and 18.2m spacing of tiles. Tile flow was considerably greater for the 9.1 m spacings. Plamenac(1988) plotted discharge rates of tiles at various spacings in a heavy clay soil. Thoughmaximum discharge volumes were similar, tiles that were further apart took longer todrain. Tiles 20 m apart took about 40 hours to drain, whereas at 50 m spacing, drainagetook about 125 hours.

d) Stream flowResearchers at the University of Waterloo examined stream flows from the middleThames River in Ontario from 1949-1980. Their results showed that drainage had little tono effect on the stream flow for the watershed for the period observed (Serrano et al.1985). Eddie (1982) also failed to find any trend in stream flow characteristics that might


be related to agricultural drainage. The study examined the annual mean, maximum andminimum stream flows of 10 rivers in western Ontario.

e) GroundwaterFrom a study that collected tile drainage data for 16 years from a site in central New York,Walter et al. (1977) found that only 7.5% of the average annual precipitation came out inthe tile discharge. This study also found that most of the average annual tile flow (84%)occurred during the months from November to April. This is during periods of abundantmoisture, where water would be lost in runoff anyway. The driest months were from Julyto September, during a time when very little tile flow would be occurring. Soil properties,especially hydraulic conductivity, can affect the extent to which land drainage can affectground water levels (Hill 1976).

4.2 Water QualityWater quality, as it relates to agricultural practices, is an important issue. This section willexamine how potential water contaminants are affected by tile drainage.

a) SedimentWall et al. (1982) determined that 0 to 30% of suspended sediment in streams comes frombank erosion, and 70 to 100% comes from cropland sheet erosion. These numbers weredetermined by studying 11 small (<6 000 ha) agricultural watersheds over a two yearperiod. Because of the additional storage volume that tile drainage can create within thesoil profile, tile drainage greatly reduces the amount of destructive overland flow from afield, and thus the amount of sediments lost. Baker and Johnson (1976); Hill (1976);Loudon et al. (1986); Belcher and Fogiel (1991); Skaggs et al. (1994); and Thomas et al.(1995) all reported that tile drainage can reduce the amount of sediment lost from anagricultural watershed.

Some chemicals and nutrients, such as pesticides (mostly herbicides) and phosphorus (aconstituent of phosphate), are strongly adsorbed to soil constituents. By reducing theamount of sediments lost from a watershed, associated chemical and nutrient losses canalso be reduced (Gaynor et al. 1995). Loudon et al. (1986) found that tile drainage is aneffective method of reducing non-point source pollution in areas where sediment andphosphorus are the major concerns. Skaggs et al. (1982) also determined that tiledrainage should be considered a best management practice for reducing erosion onrelatively flat land. This recommendation was made after it was found that tile drainagecould reduce the amount of soil lost to erosion by a factor of ten on a Goldsboro sandyloam, with a 2% slope in North Carolina.

Soil erosion from tile drained fields is further reduced by tile drainage because a) tile


drainage improves soil structure, and therefore makes soil more stable and less likely toerode (McLean and Schwab 1982; Hundle et al. 1976); and b) spring planting can be doneearlier. Plant growth can sooner provide protection from wind erosion (Colwell 1978).

b) PesticidesThough tile drainage can act as a conduit for pesticides, it may actually reduce the amountof pesticides that arrive to the surface water by reducing runoff volumes. A studyconducted in the Yamaska river basin in Quebec determined that, of the total amount ofthe herbicide, atrazine, lost to runoff, 51 to 62% was removed from the field throughsurface runoff compared to16 to 24% through tile drainage (Muir and Baker, 1976). Bastien et al. (1990) also found pesticide loading to be far higher in surface runoff than insubsurface flow. Loadings in the tile drains were up to 3.47 ìg/L, compared to as much as47 ìg/L in surface runoff.

Various studies have been done to examine the importance of a variety of factors in themovement of pesticides through the soil:

$ climate and season (Bastien et al. 1990; Traub-Eberhard et al. 1995)$ pesticide sorption coefficient (Kladivko et al. 1991; Traub-Eberhard et al.

1995)$ soil type (Traub-Eberhard et al. 1995; Novak et al. 2001)$ preferential flow through soil macropores (Kladivko et al. 1991; Kladivko

et al. 1999)$ tillage practices (Elliot et al.2000)

After reviewing the literature, Gaynor et al. (1995) summarized that the highestconcentrations of pesticide in surface runoff or tile discharge typically occur in eventssoon after herbicide application. The risk of contamination is greatest: a) if there isprecipitation shortly after application, or b) on fields with slopes greater than 3%, and c)where pesticide application rates are high. On level ground, more pesticides were lostfrom surface drainage than from tile drainage.

It is important to note that tile drainage can improve crop quality (Colwell 1978). Higherquality crops are better able to withstand disease, which could result in a reduced need forpesticides.

c) Manure applicationIn Ontario, Best Management Practices (BMPs) have been produced that address theenvironmental impacts of the agricultural sector. The following are somerecommendations for minimizing the risk of tile drainage contamination from manure (orfertilizers):


$ consider risks to water resources before application - make sure crops can usemanures or fertilizers at time of application;

$ consider cultivation to break up preferential flow paths on the soil surface andwithin the soil before applying liquid fertilizer or liquid manure;

$ monitor irrigation and tile systems carefully, especially if applying fertilizers ormanures

$ avoid winter manure application, especially on sloping land; and$ do not apply manure on saturated soils. (Ag Can and OMAF 1994)

Fleming and Bradshaw (1992) found that working the soil prior to spreading of manureminimizes the amount of contamination from the manure. They also recommended thatfarmers should monitor tile drainage outlets during and after manure application. Tilewater contamination was easily detected as it would run darker and have a noticeableodour. Geohring et al. (1998) found that spreading liquid manure when the soil was dryreduced the risk of macropore flow.

d) NutrientsStudies examining the nutrient losses from tile drain effluent are many. The generalconsensus in the literature, is that tile drainage reduces P, potassium (K), organic nitrogenand ammonium (NH

4) losses (Baker and Johnson 1976; Hill 1976; Bengtson et al. 1982;

Belcher and Fogiel 1991; Konyha et al. 1992; Skaggs et al. 1994; and Thomas et al. 1995). The studies show that this is largely because sediment-associated nutrients, such as P andK, do not move easily through the soil profile. With tile drainage reducing runoffvolumes, less of these nutrients are transported to surface waters. Nitrates (NO

3), on the

other hand, are highly mobile in the soil because they easily dissolve in water. Variousreviews show elevated losses of nitrogen from tile drained fields compared to undrainedfields (Baker and Johnson 1976; Hill 1976; Bengtson et al. 1992; Belcher and Fogiel 1991;Skaggs et al. 1994; and Thomas et al. 1995).

A study by Bengtson et al. (1982) compared water quality from four plots. Two had tiledrainage and two did not. In the tile drained plots, surface runoff and soil losses wereboth lower by 17%. Total P and K losses were lower by 48% and 22%, respectively. Only total N values were higher - by 3.2%. Further study of this site was conducted byBengtson et al. (1992). After an additional 10 years, average runoff volume, soil, P and Klosses were lower by 35, 31, 31 and 27%, respectively. These reductions occurred despitea 27% increase in water leaving the site. The site was a 4.4 ha Commerce clay loam on a0.1% slope, near Baton Rouge, Louisiana.

Rooting systems of crops can affect nutrient losses from tile drains. Shallow rooted


crops, like the potato, often have high leachate losses in tile water (Milburn et al.1990;Madramootoo et al. 1992b). This is because shallow roots are unable to remove nitratesfrom greater depths in the soil, leaving them to be leached away. Because tile drainageremoves excess water from the upper layers of the soil more quickly than undrained soils,plants in drained fields often develop deeper rooting systems. As well, drainage promotesbeneficial soil bacteria activity and improves soil tilth (Van Vlack and Norton 1944). Thisalso allow the roots to penetrate deeper into the soil. Deeper roots means more nutrientsand water can be accessed for plant use. Improved soil tilth also increases soil aerationwhich allows for increased nutrient availability through bacterial activity. If nutrients areavailable to plants and bacteria, they are less apt to leach downward through the soilprofile.

Timing of when drainage occurs can affect the amount of nutrients lost. During dryconditions, total phosphorus in the soil can accumulate as microbes break down organicphosphorus. If there is a heavy rainfall, some of this P will be essentially "flushed" out ofthe soil profile. According to Simard et al. (2000), this can happen to a greater degreewith permanent grasslands, partly due to increased preferential flow. A study by Hooda etal. (1999) with intensively managed tile drained pastures supported this finding.

Soil type has an influence on the risk of phosphorus leaching. More P has been shown toleach from organic soils compared to mineral soils, largely dependant on the amount ofadsorbed cations in the soil, particularly aluminum, as well as the pH of the system (Miller1979).

Yearly precipitation patterns can have an impact on long-term nutrient loadings in tiledrains. High levels of residual nitrate-N can accumulate in the upper layers of the soilduring dry years when drainage does not occur. This nitrate will be released duringsubsequent years. Randall (1998) investigated 5 different tile drainage sites over a 20 yearperiod in southern Minnesota. Following a dry year, levels of nitrate-N in tile outflowwere two to four times higher than normal levels. Elevated levels persisted for four yearsafter the dry year. Randall urges that scientists and citizens and farmers shouldunderstand these types of cycles and manage crop and nutrient inputs accordingly.

Cropping systems can also affect nutrient levels in tile drainage (Randall et al. 1997). Acorn-soybean rotation resulted in lower nitrate losses than in plots grown undercontinuous corn (Kanwar et al. 1999). Neilsen et al. (1980) monitored eleven agriculturalwatersheds in the Great Lakes region from 1975 to 1977. Elevated levels of dissolvednitrogen (NO

3+ NH4) were often highest in well-drained watersheds growing corn, with

high fertilizer use. Bolton et al. (1970) found N, P, and K losses were greatest under corn


and least under a bluegrass sod. After studying the tile discharge from 20 farms insouthwestern Ontario, Fleming et al. (1998) found nitrate-N, total P, and K concentrationsto be greater under field or seed corn crops (mean concentrations were 18.4, 1.6 and 7.6mg/L, respectively) when compared to soybean, tomato or wheat crops. The OntarioDrinking Water Standard for nitrate-N is 10 mg/L (there is currently no standard fornitrate for surface water quality).

Various researchers have studied the seasonal impact on tile water nutrient levels.Bjorneberg et al. (1996) found that 45 to 85% of the annual nitrate-N losses throughsubsurface drainage occurred in the spring and fall, corresponding with times when cropswere not growing, as well as with changes in rainfall and drainflow. This trend was seenbelow four different tillage systems: chisel plow and moldboard plow, ridge till andno-till. Nitrate losses were decreased under ridge till and no-till, compared to chisel plowand moldboard plow. They emphasized that the mineralization of inorganic nitrogen canoccur throughout the year, not just when fertilizer is applied.

5.0 Management Systems

Management systems exist that can reduce the amount of contamination in tile drainage,when it does occur. This section will briefly outline some of the systems in use today.

5.1 Nutrient Management PlanningBy applying nutrients and chemicals to the land on a needs-basis as well as at appropriatetimes, tile water contamination can be greatly minimized (Ritter and Chirnside 1987;Parkes et al. 1997). This is currently the subject of a major shift in management inOntario. 5.2 Water table managementTwo types of water table management, controlled drainage and sub-irrigation, seem to bequite promising methods of reducing nitrogen losses in tile water (Meek et al. 1970; Dotyet al. 1986; Belcher and Fogiel 1991; Madramootoo et al. 1992a; Madramootoo 1994;Drury et al. 1996; Tan et al. 1998; Tan et al. 1999; ). These have application where the landis fairly level. Controlled drainage contains a mechanism that keeps the water table at a setelevation, stopping drainage water from exiting the system. Sub-irrigation is similar butincludes a method to introduce fresh water to the system in order to maintain the watertable elevation. These systems increase denitrification rates in the root zone, whichreleases N to the air that otherwise would reach the groundwater or surface water.

Drury et al. (1996) found controlled drainage could reduce average annual nitrate losses


by 43% from 25.8 to 14.6 kg/ha, compared to drainage treatments. In conjunction withconservation tillage, nitrate losses were reduced by 49% (11.6 kg/ha). Nitrate losses fromsurface flow increased in controlled drainage, but this amount was minimal (1.4 kg/ha). Eighty-eight to 95% of the NO3 losses occurred during the non-crop period (Nov. toApr.). This study was conducted on a Brookston clay loam.

Compared to free outlet tile drainage, Tan et al. (1999) found flow weighted mean nitrateoutflow concentrations were reduced by 38% and total N losses were reduced by 37%with controlled drainage. On this sandy-loam soil, tomato and corn yields were shown toimprove by 11 and 64%, respectively, on the site with controlled drainage. Improvementsin crop yield due to controlled drainage were also reported by Doty et al. (1986); Nemonet al. (1987); and Madramootoo et al. (1992a).

Controlled water tables can also reduce concentrations of the herbicide atrazine in shallowgroundwater. This is what Kalita et al. (1992) found from a Nicollet silt-loam with a 2.5%slope.

5.3 Constructed wetlandsPublic perception of wetland "utility" in the past resulted in many natural wetlands beingconverted into agricultural land. Now, natural wetlands have been recognized for theirecological role in filtration, habitat and groundwater restoration. The Federal WetlandsPolicy, enacted in Canada in 1991, classifies wetlands and protects them from conversionaccordingly.

AConstructed@ wetlands have similar benefits to the environment and are being exploredas a mechanism to treat wastewater from livestock farms and agricultural drainage. Though long-term studies are still needed to establish the full impact of such wetlands(Woltemade 2000), current research indicates constructed wetlands are effective atremoving nutrients, sediments and chemicals from agricultural wastewater. A simulationmodel based on data collected in field experiments predicted that wetlands can treatagricultural drainage water (Chescheir et al. 1992). It was predicted that the wetlandbuffer could remove 79% total Kjeldahl nitrogen (TKN), and 82, 81 and 92% of the NO3 ,total Phosphorous and sediment, respectively, over a 20 year period. Long narrow bufferareas were more effective than short wide wetlands because of higher residence times.

Brown et al. (1998) found that a constructed wetland can double as a treatment systemand an irrigation reservoir. The benefits of the whole system were multiple: supply waterto crops; eliminate drought stress; improve plant nutrient use; sustain yields; collect andrecycle runoff and drainage; reduce off-site and downstream impacts; reduce amount ofsediment and plant nutrients lost from cropland to surface waters; increase wetland acres,


vegetation, and wildlife habitat.

5.4 OtherOther methods of management which can reduce the risk of tile drainage contaminationinclude:$ marking outlets so they can be found at all times of the year;$ use of control valves to shut off the system if a manure leak is detected;$ use of inspection chambers on tile drains at farm lot lines to allow farmers to see

and take samples of tile drain water;$ education about the hazards of illegally connecting milkhouse washwater

treatment systems or household septic systems to field tile drainage systems;$ setting up a regular sampling schedule, complete with detailed records, of tile drain

water quality, with tests for bacteria (especially if it is a livestock farm), nitrate,phosphorus and other potential contaminants; and

$ creation of contingency plans - what to do if a problem arises, who to contact, etc.

6.0 Summary and Conclusions

Tile drainage is both agronomically and economically beneficial for reasons includingbetter growing conditions, improved soil structure, better trafficability, reduced energyconsumption, more timely planting and harvest, and improved yields for a variety ofcrops. Tile drainage can also impact the environment. After reviewing a large number oftile drainage research works, the following conclusions can be made:$ Rather than rely on evaporation alone to remove excess water from the soil, tile

drainage can remove excess water within a matter of days rather than weeks. As aresult, tile drained soil has increased water storage capacity.

$ Soils with increased storage capacity, such as tile drained soils, have a higherinfiltration capacity. Higher infiltration means a) the soil acts as a buffer torainfall, decreasing stream peak flow volumes and extending watershed totalrunoff over a longer period of time; and b) surface runoff volumes can be reduced.

$ The degree to which peak flows are affected by tile drainage depends on themoisture conditions in the soil. When soil is dry prior to rainfall, peak flows canbe reduced by tile drainage by as much as 87%. Soil type, slope and drainagespacing also affect infiltration and peak flows.

$ Tile drainage increases annual total runoff volumes compared to surface drainageonly.

$ Between 70 and 100% of suspended sediment in streams comes from croplandsheet erosion. Due to decreased surface runoff, tile drainage decreases sediment


loading in streams by as much as 40%. Because crops can be planted and thusprovide protection to the soil earlier in the spring, tile drainage also reduces themagnitude of wind erosion on soil.

$ P and K do not move easily through the soil profile, so reduced surface runofffrom tile drainage also reduces total P and K losses in mineral soils by as much as48 and 29%, respectively.

$ Conditions which lead to the greatest nitrate losses in tile outflow are: a) when tilewater flow is greatest; b) under corn-cropping systems; c) after an extended dryperiod; d) where fertilizer use is high (possibly as a result of poor nutrientmanagement planning).

$ Because of a reduction in surface runoff, total pesticide losses from tile drainedfields are also reduced compared to surface drained fields.

$ Nutrient management plans, water table management and constructed wetlandsare management systems which can be used to correct problems with tile waterquality, where they exist.

There are many environmental benefits of tile drainage systems on farms. In those caseswhere contaminated water exits from tile drainage systems, it is usually the result of apractice over which the farmer has control. If this happens, there are strategies that thefarmer can use to either prevent contamination from entering the tile drains, or treating thewater at the outlet. When used properly, tile drainage is an environmentally responsiblepractice.


7.0 References

Ag Can and OMAF. 1994. Best Management Practices: Water Management. AgricultureCanada and the Ontario Ministry of Agriculture and Food. Governmentpublication.

Aldabagh, A.S.Y. and Beer, C.E. 1975. Economics of Increased Mobility from TileDrainage. Transactions of the ASAE, St. Joseph, MI.

Bailey, A.D. 1979. Benefits from drainage of clay soils. Paper No. 79-2549, St. Joseph,MI.

Baker J.L. and Johnson, H.P. 1976. Impact of subsurface drainage on water quality.Proceedings from the 3rd National Drainage Symposium, Chicago, Ill.

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